1. Field of the Invention
The present invention relates to a steel for machine structural use that has excellent machinability and is used as a raw material for industrial equipment, automobile components and the like.
2. Description of the Prior Art
Steel products that are machined for use in industrial machinery, automobile components and the like need to have excellent machinability. As steels for machine structural use that have excellent machinability, sulfur free-cutting steel containing at least a certain level of sulfur and lead free-cutting steel containing a small amount of lead are specified in the Japanese Industrial Standards (JIS). In addition, free-cutting steels containing elements having properties similar to those of lead such as Bi, Te or Se have been developed, but have not become widespread in industry for reasons such as high price.
The steel for which results in terms of machinability can be expected with most certainty is lead free-cutting steel, and a most significant characteristic feature of this steel has been that the mechanical properties of the steel are not degraded even though lead is contained therein. However, during the process of manufacturing lead free-cutting steel and the process of cutting or turning the steel material, lead is scattered into the air as fumes, thus degrading the working environment and, moreover, when disposing of industrial waste generated through these processes such as slag and chips, problems arise in terms of environmental protection due to the steel containing lead.
On the other hand, regarding sulfur free-cutting steel, which has the longest history as free-cutting steel, there are great variations in terms of the form and distribution of sulfides in industrially manufactured steel and hence the reliability with regard to the machinability has been poor. There has also been the problem that if one attempts to improve the machinability by making the sulfur content high, then hot brittleness may occur during the process of manufacturing the steel material, resulting in many defective articles.
Nevertheless, unlike lead, sulfur has few problems in terms of health and safety, environmental issues and so on, and hence people have been awaiting the development of a sulfur free-cutting steel that does not contain lead but has a machinability on a par with conventional lead-containing free-cutting steel. It is thus an object of the present invention to provide a sulfur-containing free-cutting steel for machine structural use that has excellent machinability.